This invention relates to novel individual particle crystals of tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxymethyl]methane (hereinafter sometimes referred to as Compound-A) in an .alpha.-crystal form which is useful as an antioxidant for polyolefins and the like. In further detail, it relates to individual particle crystals of Compound-A having .alpha.-crystal form, comprising crystals in substantially cubic or rectangular parallelepipedal shape or aggregates thereof.
The products of Compound-A marketed presently have critical deflects in workability, measurability and environmental hygiene since they are inferior in powder-properties, for example, in that they are in the fine powder form, of low bulk density and of inferior fluidity and easily scatter and drift in handling. Therefore, improvement is desired.
Compound-A has a property crystallographically called multiple forms, which means that it has various stable or semi-stable crystal forms representable by the same chemical structure. Heretofore, there have been known several crystal forms defined as .alpha.-form, .beta.-form, .gamma.-form, .delta.-form and the like. The above-mentioned marketed products are in the .beta.-crystal forms. There is close relationship between the respective crystal forms and the purity and powder property (micromeritical property) of Compound-A. By a variety of methods, the objective crystal forms can be obtained.
Hitherto, there have been presented proposals for improving the defects of the marketed products by preparing Compound-A in particular crystal forms or by obtaining Compound-A by combination of a particular crystal form with a particular particle form. For example, (1) in the official gazette publications of Japanese Patent Applications Publication (Kokoku) Nos. 13018/1985 and 13017/1985, the powder property, which is a defect of .alpha.-crystal and the marketed .beta.-crystal, is improved by obtaining Compound-A in the .delta.-form which is a semi-stable crystal form and (2) in the official gazette publication of Japanese Patent Application Laid-open (Kokai) No. 258343/1987, are improved the defects of the marketed products by obtaining Compound-A in individual particle crystal forms by recrystallizing under special conditions, though in the same semi-stable .beta.-form as the form of the marketed products.
On the other hand, the crystals in the .alpha.-form which belongs to the stable crystal forms, can be obtained by recrystallization from a solvent such as n-heptane, n-hexane or methanol as disclosed in the offical gazette publications of Japanese Patent Applications Publication (Kokoku) Nos. 18617/1967 and 19083/1967, Japanese Patent Application Laid-open (Kokai) No. 156645/1985 and so on. However, the case is that these crystals are the most inferior both in purity and in powder property, and therefore are less valuable products.
As mentioned above, the heretofore proposed methods for improvement relate to preparing Compound-A in the .delta.-form and in the individual particle .beta.-form, both of which are unstable crystal forms, avoiding the .alpha.-form which is inferior in purity and powder property, for the purpose of obtaining Compound-A having superior powder property. These methods, however, cannot be looked upon as economical in that the number of steps is increased and a large amount of impurity is allowed to exist.
That is, according to the methods of Japanese Patent Applications Publication (Kokoku) Nos. 13017/1985 and 13018/1985, in the ester interchange reaction, it is necessary to add a dicarboxylic acid ester of the formula (1) ##STR1## wherein R.sup.1 and R.sup.3 respectively stand for an alkyl group having 1-6 carbon atoms and it is also necessary to isolate the molecular adduct of Compound-A with isopropanol, which results in the increase in the number of the steps. Thus, the methods are not economical. In the method of Japanese Patent Application Kokai No. 258343 1987, it is necessary to allow methyl or ethyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, which is a starting compound, and the tris-substituted compound of the formula (2) ##STR2## which is an intermediate compound resulting from incomplete reaction, to be contained in a large amount before purification by recrystallization (as described in the specification, when the content of the objective Compound-A is too high at the completion of the reaction, it is required to add the starting compound or the tris-substituted compound (2) and purposely mix them therewith). Thus, the obtained products tend to have reduced purity and also the yield of the recrystallization is lowered, and the method is not an economical one.